Try making an anonymous array like so:

$files[$i]{hashkey}= [ split(/,/,$ARGV[0]) ];
[download]

those extra square brackets will create an array containing the results of split and return a reference to it. now you'll access an element like so:

print $files[$i]{hashkey}->[0];
[download]

where -> is the dereference operator and 0 is the index of the element in question.